To move small components, electromagnetic motors are often used because they are relatively inexpensive. The electromagnetic motors rotate very quickly and can only apply a low force, so they are always used with a gearbox that provides the slower motion and increased power necessary for practical applications. It should be noted that the movement of driven elements referred to in this disclosure refers to a translation or rotary motion in a common direction, and does not included motion that merely moves a part alternatively back and forth to shake the part without any net movement. While the conventional electromagnetic motors are relatively inexpensive, there are a large number of moving parts which complicates assembly and reliability, and the low power and need for a gearbox not only limits their application but also makes the cost excessive for many applications. Moreover, these motors are too big, not very precise in their motion, and are noisy. There is thus a need for a simpler, quieter and less expensive motor.
An alternative type of small motor is a piezoelectric motor, which uses a material that can change dimension when a voltage is applied to the material. Piezoelectric ceramics are used in electromechanical micromotors to provide linear or circular motion by making frictional contact between the vibratory motor and a driven object. These piezoelectric motors are composed of at least one mechanical resonator and at least one piezoelectric actuator. When electrically excited by oscillating electrical signals, the actuator generates mechanical vibrations that are amplified by the resonator. When the resonator is brought into contact with a body, these vibrations generate frictional forces in the contact area with the body and cause the body to move. The speed, direction and mechanical power of the resulting mechanical output depend on the form and frequency of the vibrations in the contact area. These piezoelectric motors work with small changes in dimension for a given voltage, and they can vibrate at many tens of thousands of cycles per second. Various cumbersome and expensive designs have been used to obtain useful forces and motions from these small vibratory motions.
One type of piezoelectric motor is a traveling wave motor, which uses a wave that travels through the piezoelectric material. These motors typically are based on a disc shaped design and are expensive to produce. The shape and the cost of these motors limit their application.
Other types of piezoelectric motors require a specially shaped waveform in the input signal in order to cause the piezoelectric material to move in a desired direction. One such type of motor is referred to as a stick-slip drive. These motors have a piezoelectric element that moves an object in a desired direction on a support at a relatively slow rate sufficient to allow friction to move the object. The waveform applied to the piezoelectric element causes the piezoelectric to then quickly retract and effectively pull the support out from under the object causing the object to slip relative to the support. The process is repeated, resulting in motion. Since these motors require a sawtooth or similar shaped waveform to operate, they require complex electronics that increase the cost of such motors.
A yet further type of piezoelectric motor is the impact drive, which repeatedly hits an object in order to make it move.
In piezoelectric micromotors, the piezoelectric element can be used to excite two independent modes of vibration in the resonator. Each mode causes the contact area on the resonator to oscillate along a certain direction. The modes are often selected so that the respective directions of oscillation are perpendicular to each other. The superposition of the two perpendicular vibrations cause the contact area to move along curves known as Lissajous figures. For example, if both vibrations have the same frequency and no relative phase shift between the vibrations, the motion resulting from the superposition is linear. If the frequencies are the same and the relative phase shift is 90 degrees, then the resulting motion is circular if the amplitudes of each vibration are identical; otherwise the resulting motion is elliptical. If the frequencies are different, then other motions such as figure-eights can be achieved.
The Lissajous figures have been used to produce figure-eight motion drives. These drives require an electrical signal that has to contain two frequencies to cause a tip of the vibration element to move in a figure-eight shaped motion. The resulting electronics are complex and expensive, and it is difficult to use the figure-eight motion to create useful motion of an object.
In order to move another body and to create a mechanical output, circular or large-angle elliptical motions (semi-axes nearly equal) have been preferred over linear motions. Piezoelectric micromotors in the prior art thus commonly employ two perpendicular modes of vibration that have a relative phase shift of approximately ninety degrees. The modes are excited close to their respective resonance frequencies so that the resulting mechanical output is maximized. If the relative phase shift between the two modes is changed to −90 degrees, the direction in which the ellipse is traversed is reversed. The motion of the body in contact with the resonator is thus reversed as well. But these conventional motors require two piezoelectric drivers located and selected to excite the two separate resonant modes. This requires two sets of drivers, two sets of electronic driving systems, an electronic system that will reverse the phase of each driver, and the basic design places limitations on the locations of components.
The prior art thus includes electromechanical micromotors where a rod-like resonator has a small piezoelectric plate that is attached to the resonator. The resonator contacts the moving body at the tip of the rod. The actuator excites a longitudinal mode and a bending mode of the rod. The excitation frequency is chosen in-between the two resonance frequencies of the respective modes so that the relative phase shift is 90 degrees. The phase shift is generated by the mechanical properties of the resonator, in particular its mechanical damping properties. The resulting elliptical motion of the resonator's tip is such that one of the semi-axes of the ellipse is aligned with the rod-axis and the other semi-axis of the ellipse is perpendicular thereto. A second piezoelectric actuator is used to reverse the direction in which the ellipse is traversed, and is placed at a different location on the resonator. The second piezoelectric actuator is located in such a way that it excites the same two modes but with a relative phase shift of −90 degrees.
Unfortunately, this actuator requires two sets of electronics to drive the motor in opposing directions, and has two sets of driving piezoelectric plates, resulting not only in a large number of parts but also greatly increasing the complexity of the system and resulting in significant costs for these type of motors. The motor also has limited power because the driving frequency is selected to be between two resonant frequencies. There is thus a need for a vibratory motor with simpler electronics, fewer parts, and greater efficiency.
In other vibratory motors, a piezoelectric element has a number of electrodes placed on different portions of the element in order to distort the element in various ways. Thus, for example, two modes of vibration can be excited by at least two separate, independently excited electrodes in each of four quadrants of a rectangular piezoelectric ceramic element. A second set of electrodes is used to reverse the direction in which the ellipse is traversed. The resulting elliptical motion is such that one of the semi-axes of the ellipse is aligned with the longitudinal axis of the motor and the other semi-axes of the ellipse is perpendicular thereto. As mentioned elsewhere, the ratio of the semi-axes can be advantageously used to increase motion or reduce travel time, by making advantageous use of ratios of 5:1, 10:1, or from 20-50:1. Again though, there are a number of electronic connections and many parts to achieve this motion, resulting in a high cost for this type of motor. It is an object of some aspects of the present invention to provide a micromotor, which is cheaper and easier to manufacture than previous art.